I doubt that you'll find anyone on AB who's ever got a micrometer out to measure the thickness of their rubbish bags ;-)
So you can only go on supplier's descriptions.. This site describes 120 gauge as 'economical/light duty, for general office waste'. (I'm guessing that they'll be like the 'value' bin bags in supermarkets). 160 gauge is 'general purpose, for kitchen waste). (Another guess: The same as 'standard' bin bags in supermarkets?). Their wheelie bin liners are 140 gauge. But for 'extra heavy duty, for garden use' they offer 300 gauge:
http://www.washingnet.org.uk/plastic_bags.html
If you're just looking for strong 'bin bags' (for the uses that people normally put them to), look for around 160 gauge. But if you need anything stronger, forget labels like 'bin liners' and 'refuse sacks'. Look for 'garden refuse bags' or 'rubble sacks' (which that site shows as 500 gauge).
Chris